1. Field of the Invention
The present invention relates to a nitride semiconductor laser element, and more particularly relates to a nitride semiconductor laser element having a ridge waveguide structure.
2. Background Information
A nitride semiconductor is formed from a semiconductor compound containing Inx Aly Ga1-x-yN (0≦x, 0≦x+y≦1), and there has been growing demand for semiconductor laser elements that make use of this material in optical disk systems that allow large quantities of information to be recorded and reproduced in high density (such as next-generation DVDs), and in personal computers and other such electronic devices. Considerable research has therefore gone into semiconductor laser elements that make use of nitride semiconductors.
Also, it is believed that semiconductor laser elements that make use of nitride semiconductors are capable of emission over a wide range of visible light wavelengths, from ultraviolet to red, so such elements are expected to find use in many different applications, such as laser printers, laser displays, optical network light sources, and so forth.
In particular, there have been various studies into the structure of semiconductor laser elements, and there have been proposals for structures that allow favorable lateral mode control, structures that affords lower power consumption, higher output, higher reliability, smaller size, longer service life, etc., and so forth. Of these, a structure that holds particular promise is one with a ridge waveguide structure. With a semiconductor laser element having such a structure, a protective film is usually formed from the side faces of the ridge to the surface of the nitride semiconductor layer on both sides of the ridge, in order to achieved optical confinement (see, for example, Japanese Laid-Open Patent Application H09-283845, H10-270792 and 2006-24703).
An insulating protective film such as this is selected after taking into account such factors as adhesion and the difference in refractive index from that of the nitride semiconductor layer.
However, even when a protective film is selected by taking the refractive index into account, this protective film may not necessarily have good adhesion with the nitride semiconductor layer. Also, even if a protective film with good adhesion to the nitride semiconductor layer is selected, optical confinement may not be good. Therefore, forming a protective film that continues to function well throughout extended continuous drive in terms of both adhesion and optical confinement is necessary for a nitride semiconductor laser element.